Pneumatic rubber tires are conventionally prepared with a rubber tread which can be a blend of various rubbers which are, typically, sulfur curable or sulfur cured as the case may be, diene based elastomers. The tire rubber, including its tread portion, is typically reinforced with carbon black reinforcing filler and with a minimal, if any, of silica.
In one aspect, treads of rubber tires are often prepared of a cap/base construction in which the outer portion of the tread is the cap and the underlying part of the tread between the tread cap and the supporting tire carcass, is its base. The cap portion is usually designed to be ground-contacting and, thus, have associated properties and the base portion generally overlays the tire carcass and is usually designed to support the cap, therefore, not to be ground-contacting. Such cap/base constructions are well known to those skilled in the art.
In one aspect of this invention, a tread of cap/base construction is contemplated in which, for the purposes of this invention, the tread cap is substantially reinforced with silica and the underlying tread base is substantially carbon black reinforced.
In one aspect, the tire tread, or tread base in the case of a cap/base construction may be presented in a form of a base-wing in which a portion of the tread, or tread base as the case may be, extends outward and over a portion of the outer surface of the tire sidewall of the tire carcass.
It is to be appreciated that conventional tread, and tread cap/base, as the case may be, constructions are typically prepared by an extrusion process where, in the case of a cap/base construction, the cap and base are extruded together to form a unified extrusion. Such tread extrusion processes are well known to those having skill in such art.
For the purposes of this presentation, a tire is viewed as being composed of a circumferential tread and supporting carcass therefor. The carcass is viewed as being composed of relatively conventional elements which include but are not limited to carbon black reinforced rubber sidewalls (although a portion of an outer surface of the sidewalls may be colored by an appropriate pigment such as, for example, white titanium dioxide and, thus, not contain carbon black), beads, apex, innerliner and the supporting carcass plies, including fabric reinforced plies. A shoulder region of a tire is considered as being a portion of the tire where its sidewall meets its tread. It is not normally a sharp line of demarkation and its actual position may vary somewhat from tire to tire. The bead portion of the carcass is typically composed of a relatively inextensible bundle of wires which is encased in carbon black reinforced rubber and is designed to contact a metal rim on which the tire itself is mounted to form a tire/rim assembly which itself is conventionally adapted to be mounted on a vehicle, particularly a wheel of a vehicle. The rim is typically steel or aluminum, or alloy thereof and is, thus, electrically conductive since the metal is considered as having a very low resistance to flow of electricity. The term "metal", as used herein for the metal rim, is intended to mean electrically conductive metals such as, for example, the aforesaid steel and aluminum rims as would be understood by those having skill in such art.
By the term "carbon black" reinforced, it is meant that the rubber components of the tire carcass rubber which are carbon black reinforced, contain a quantitative amount of carbon black reinforcement, normally at least 25 phr, and a minimal amount, if any, of silica and the weight ratio of carbon black to silica is at least 5/1.
It is acknowledged that, in some tire constructions, carbon black reinforced rubber components such as, for example, components which are sometimes known as chippers and chafers may be positioned in the bead area or region of the tire construction to assist in cushioning the bead component against the metal rim. In the context of this description, a reference to the aforesaid bead component of the tire carcass is intended to include such other associated rubber components unless otherwise indicated, and thereby are a part of the tire carcass.
In practice, as is well known to those having skill in such art, the tire, which may sometimes be referred to as a pneumatic tire, is mounted on the metal rim and air pressure is applied to the cavity enveloped by the metal rim and the pneumatic tire carcass.
The hereinbefore construction elements, or components, of a pneumatic tire and tire carcass, as well as such tire/rim assembly, are also well known to those familiar with such tire art.
It is important to appreciate that uncompounded rubber by itself is generally considered as being, substantially, an electrical insulator or, in other words, a rather poor conductor of electricity.
A carbon black reinforced rubber vehicular tire, while still providing a degree of resistance to flow of electricity, has a considerably higher electrical conductivity, or lower resistance to flow of electricity, than rubber without the carbon black reinforcement.
It is considered herein that a continuous relatively low electrical resistance path is created between the electrically conductive metal rim of a tire/wheel (tire/rim) assembly to the outer tire tread surface, and thence to the ground via the carbon black reinforced rubber of the tire, including its ground-contacting tread component, for such a tire/rim assembly mounted on a vehicle which is intended to travel over the ground.
In this manner, it is considered herein that potential electrical energy, which may potentially be created by components of or within a moving vehicle as its rotating wheels and associated tire/rim assemblies as they travel over the ground, is dissipated from the rim of a tire/rim assembly on the vehicle to the ground via the carbon black reinforced rubber path of the tire carcass and tread, or tread cap of a tread of a cap/base construction, which tread or tread cap, as the case may be, is normally the outer rubber surface of the tire intended to be ground contacting.
Thus, in one aspect, it is considered herein that the carbon black reinforced rubber of the tire carcass and associated tread normally provide a sufficiently low electrical resistance path to dissipate potential electrical energy and, thereby, retard or eliminate static electrical charge from building up and/or accumulating under dynamic conditions of a rotating tire on a vehicle traveling across the ground.
Alternatively, in practice, carbon black reinforced rubber tires may sometimes be prepared which have outer rubber treads designed to be ground contacting which are quantitatively reinforced with silica and, thus, contain only minimal amounts such as, for example, 15 phr or less, of carbon black.
In such silica reinforced tire tread construction, although the various other rubber components of the tire, namely, the aforesaid overall tire carcass, are quantitatively reinforced with carbon black with a minimal amount, if any, of silica and, thus, may have a relatively low electrical resistance on the order of one megohm or less whereas, the silica reinforced tread itself may have a substantially high electrical resistance on an order of at least one thousand megohms and, thus, such tread creates a degree of electrical insulating effect between the tire carcass and ground. Such a tire construction has a substantially less tendency to dissipate static electricity from the tire to the ground, and particularly from the metal rim of a tire/rim assembly to the outer surface of the tire tread and thence to the ground, which may be generated by a dynamic condition of the rotation of the tire on a moving vehicle. Accordingly, a potential for static electricity to build-up, or increase, is considered to be higher for such a tire construction with a silica reinforced tread than for a similar tire with a carbon black reinforced tread.
Therefore, it is desirable to provide a suitable path of relatively low electrical resistance between the tire bead portion and the tread outer surface for such a tire having a quantitative silica reinforced rubber tread and minimal, if any, carbon black reinforcement.
While the dissipation of generated electrical energy may not be completely understood, it is believed that, insofar as a vehicular tire is concerned, electricity may be transmitted primarily from the metal rim, of steel or aluminum, for example, thence on or through the carbon black reinforced rubber surface of the tire carcass to the outer surface of a carbon black reinforced rubber tread and thence to the ground.
It is acknowledged that it might be thought of to apply an organic solvent based or water based coating of carbon black-containing rubber composition over the outer surface of the quantitative silica reinforced rubber tread to facilitate a path of relatively low electrical resistance connecting the tire's carbon black reinforced rubber sidewall region to the outer tread surface, and thence the ground as the tire rotates on the vehicle. Indeed, elastomer/carbon black coatings, which may be applied as either water-based or organic solvent-based compositions and which are sometimes called pre-cure paints, are often applied to various surfaces of green, or unvulcanized, tire construction before the tire is vulcanized in a suitable mold. A purpose for such pre-cure paints might be, for example, to reduce friction between the tire and its associated vulcanization mold and increase air bleed between the tire and mold during the vulcanization operation. For example, see U.S. Pat. Nos. 4,857,397 and 4,329,265.
However, it is recognized that a thin outer rubber coating, if applied to a tire tread surface, will relatively quickly wear away as the tire is used, leaving the coating on surfaces within the grooves in a tire tread which typically has a lug/groove design or configuration. Thereby, it is considered herein that only a very small portion of the coating, namely the thickness of the coating on the walls of the tire tread lugs, is actually available to be directly presented to, or contact, the ground to facilitate a relatively low electrical resistance from the tire to the ground for a tire with tread which is quantitatively reinforced with silica.
It is, therefore, considered herein that in order to be practical, the carbon black reinforced tread outer top cap layer over the outer surface of the silica reinforced rubber tread should be a thin carbon black-containing rubber layer integral with the tread and, thus, having good adhesion to the tread, particularly within the tire tread grooves including the lug walls, and present a sufficient cross-sectional thickness, or surface area, to the ground of the outer top cap layer on the tire lug walls to be effective after the said layer wears away from the outer surface of tread lugs.
In one alternative aspect, for a tire tread conventionally configured with a combination of lugs and grooves, it is desired that the grooves connect directly or indirectly with the carbon black reinforced rubber shoulder of the tire, the area of the tire where the sidewall and tread meet, in order for the carbon black reinforced outer cap layer to more fully connect with the carbon black reinforced rubber portion of the tire, namely, the tire carcass and including the tread base in the case of a tread cap/base construction.
In a tire tread cap/base construction, which is well known to those having skill in such art, and for the purposes of this invention, it is envisioned that the cap is substantially silica reinforced and its base is substantially carbon black reinforced.
In practice, it is desirable that the rubber outer tread top cap layer (i) contains a quantitative amount of carbon black and is of a relatively low electrical resistance to aid in dissipation of electrical energy under the aforesaid conditions, (ii) is co-vulcanized with the rubber tire tread in order that it be integral with the tread and the walls of grooves of a tire tread configuration composed of lugs and grooves, (iii) be relatively thin so that it does not appreciably affect the tread properties of the tread, and (iv) be thick enough so a cross-section of the layer on the walls of a lug in a tread of a lug and groove configuration can present a suitably low electrical resistance to the ground so that it is not necessary to limit the selection of carbon blacks to carbon blacks with exceptionally low electrical resistance. Indeed, it is expected that the outer cap rubber composition will wear off of the outer surface of tire tread lugs during use of the tire so that the cross-section, or thickness, of the outer cap layer on the lug walls is relied upon to present a path of relatively low electrical resistance from the tread to the ground.
As used herein, the terms "quantitatively reinforced with silica", "quantitative silica reinforced rubber" and the like are generally used in conjunction with a tire tread, and with a rubber tire tread cap, in a tread cap/base construction, which contains about 30 to about 100, sometimes preferably about 30 to about 90 phr, of silica, and which may also optionally contain carbon black in which the carbon black is present in not more than about 20 phr. Often it is preferred that the ratio of silica to carbon black is at least 2/1 and sometimes at least 10/1.
The term "phr" as used herein, and according to conventional practice, refers to "parts of a respective material per 100 parts by weight of rubber". In the description herein, rubber and elastomer are used interchangeably.
In the description herein, the term "vulcanized" or "vulcanizable", may, on occasion, be used interchangeably with the terms "cured" and "curable".